Band switching arrangement for high-frequency circuits



March 15, 1949. 3, CRQCKETT 2,464,557 BAND SWITCHING ARRANGEMENT FOR HIGH-FREQUENCY CIRCUITS Filed D80- 30, 1944 INVENTOR. JOHN CROCfi'ETT ATTORNEY Patented Mar. 15, 1949 UNITED STATES RATENT OFFICE BAND SWITCHING ARRANGEMENT FOR HIGH-FREQUENCY CIRCUITS Application December 30, 1944, Serial No. 570,578

4 Claims.

This invention relates to high frequency oscillators and to a novel method of transferring the binations for the oscillator circuit. Each combination was resonated with a variable air condenser in order to produce a specific frequency, and thus over the tuning range of the variable air condenser, each combination of coils would have a certain range of frequency. Thus, the oscillator frequency of the oscillator would range between a lower value and an upper value for each of the combinations used. Customarily these combinations had their inductances so chosen that with the same condenser, a series of frequency bands were obtained.

It has been the practice in such oscillators to change the coil combination by means of a coil switch. The switch was connected in the tank circuit. Thus, each position. of the switch corresponded to a definite frequency band. Such an arrangement was termed multiple band switch- ,ing.

As it has become more and more desirable to design variable frequency oscillators with high top frequencies-frequencies in the ultra-high frequency regions-the problem of multiple band switching has become more and more difiicult .to solve. This arose primarily from the fact that in the ultra-high frequency range, the value of inductance needed is so small that the wiring of n the switch itself provides the necessary inductance. In fact, at times it becomes desirable to use an inductance which is smaller than the inductance of the wiring of the switch. In such a case the inductance of the multiple band switch provides an undesirable upper limit to the frequency which can be obtained in such a multiple band oscillator.

According to my invention this difficulty is overcome by a novel arrangement of inductors and capacitors in association with the multiple band switch. According to my invention the inductor for the highest frequency range is in the circuit all the time. However, when the multiple band switch is placed in the highest frequency range, the switch itself is not in the oscillator circuit; that is, not in the tank circuit of the oscillator. When the multiple band switch in .my invention is positioned in any other position than the very highest frequency range, the con- 2 nections are such that what was the inductance at the very high frequency becomes only a connector connecting two capacitors in parallel.

Thus, in my invention I make no attempt whatsoever to switch the high frequency inductance out of the circuit. Instead I change the connection in such a manner that the piece of conductor which is an inductance at the very highest frequency range becomes merely a connector at lower frequency ranges, and in this manner I am able to prevent the inductance of the multiple band switch from having an appreciable effect upon the ultra-high frequency range of the oscillator.

A primary object of my invention, therefore, is to provide a novel circuit arrangement by which I can eliminate the effect of the inductance of the multiple band switch for an ultra-high frequency oscillator.

Other objects and advantages of this, invention will become apparent from the description which follows, making reference to the drawing in which the figure shows the circuit diagram of my invention showing only one frequency range in addition tothe ultra-high frequency range.

In this figure the vacuum tube I is a triode oscillator tube. Grid bias is supplied with the aid of grid resistor 2 and grid blocking condenser 3. Switch 4 and switch 5 are ganged together, both being on the same shaft so that when one changes its position, the other one does likewise. In the position of the switches shown, the oscillator is connected to give the ultra-high frequency range. In this connection, power is fed to the plate from the feed supply 6 through choke 1. The variable capacitor 3 is a trimmer for capacitor H in the ultra-high frequency band. It is not in circuit in the low frequency band. The tank circuit of the oscillator is composed essentially of an inductance I!) and two capacitors II and I2. The connection of this inductor and capacitor pair thus makes the oscillator circuit essentially of the well known Colpitts form. In this form of oscillator, the frequency is determined almost entirely by the value of the inductor I0 and the equivalent capacitance of the two capacitors H and I2 in series.

It is to be noted that in this tank circuit, no part of either switch it or 5 appears as an inductance element. Consequently the inductance ii) can be made lower than it could be if a switch were imposed in the circuit of inductance I0, that is, between junctions I 3 and It where the inductor joins the two capacitors.

In order to vary the frequency over a range, it

is possible to vary both of the capacitors together. This can be done by gauging both capacitor fl and capacitor l2 on the same shaft, as is common practice in much of radio engineering techniques.

In order to shift the oscillator frequency to a lower frequency band, the switch gang 4 and 5 is moved to the other position. With these connections it is observed that choke l and capacitor 8 are not used as 'a feed to the circuit. Instead, the feed supply is inserted at l5 and inductor I6 is in the plate circuit. This time no capacitor is included. The tank circuit of the oscillator is composed of inductance coil H and the two capacitors I8 and IS. The junction point 26 is now connected to the grid feed circuit of the triode tube. This circuit now is no longer a Colpitts oscillator circuit. Instead feedback from the plate circuit to the grid circuit is obtained by the mutual inductance between inductance l6 and inductance H. The frequency of oscillation is now determined by the inductance of coil ll, the distributed capacitance of coil fl, and the equivalent capacitance of capacitor is in series with the net shunt capacitance from point 20 to ground.

This net shunt capacitance from point 2a to ground is composed of capacitor i9, capacitor l2, capacitor H, and whatever stray capacitance there is between the wiring to ground. In this connection now, it is to be noted that capacitor H and capacitor l'2 are essentially in parallel. The connection between these is now made by the connection [0 which was previously termed a high frequency inductance. However, now the frequency is so much lower than it was before that the inductance of this connection is of no primary importance. Consequently these two capacitors I l and I2 are in parallel with each other, and are, by the connection of switch 5, in parallel with capacitor l9. It is possible then to vary the frequency of the oscillator by varying capacitors l2 and H together, as by ganging them as a part of a ganged condenser. In this manner one dial again can control the frequency of the oscillator. Capacitor I9 is a trimmer capacitor, used for factory adjustment of the frequency range covered by the combination. In this circuit now, the inductance of switch It is in series with coil IE, but this is immaterial since the frequency is sufficiently low so that the inductance of the coils is high enough so that switch inductance becomes unimportant.

Tuning in both bands is done by the capacitors II and I2. This is "an important aspect of the invention, that in the high frequency band the two capacitors ll and ii! are effectively in series in the resonant circuit i'i3l l-l2, so that their effective capacitance is one-half the capacitance of either one of the capacitors (assuming that both capacitors are of equal value). This reduces the capacitance of the resonant circuit and enables the circuit to be tuned to relatively high frequencies. In the low frequency band, however, the two tuning capacitors H and i2 are effectively connected in parallel, so that their over-all capacitance is twice that of either capacitor alone. This increase in capacitance is desirable since it prevents the impedance of the tank circuit from becoming undesirably high at the lower frequencies.

In order to obtain more frequency bands, it is necessary only to put more contact points on switches 4 and 5, and to arrange other combinations of coils l6 and ll, and other combinations of condensers l8 and Hi. In this manner my invention provides a multiple band oscillator in which the highest frequency band has an inductance which is very low, and which inductance is not limited by the inductance or capacity of the multiple band switch. Although the invention has been described with respect to a specific embodiment, I prefer, however, to have the scope of the invention defined only by the following claims.

I claim:

1. In a radio frequency circuit, a tank circuit for oscillators including an inductance loop and a pair of capacitors permanently connected to opposite terminals of said loop, and effectively in series with each other whereby said tank circuit is resonated to a high frequency, an inductance coil, and switching means outside said tank circuit for connecting said inductance coil in circuit with said capacitors whereby said tank circuit is resonated to a lower frequency, at which frequency said capacitors are effectively in parallel with each other, over a circuit connection provided by said inductance loop, the reactance of said inductance loop being negligibly small at said lower frequency. a

2. In a radio frequency circuit, a tank circuit for oscillators including an inductance loop of relatively low inductance and a pair of capacitors permanently connected in series with each other and with said loop, the inductance of said loop having an appreciable value at high frequencies, the inductance loop and pair of capacitors being effectively in electrical series connection to resonate to a high frequency an inductance coil of higher inductance than said loop, and switching means outside said tank circuit for connecting said inductance coil in circuit with said capacitors said loop having substantially no inductance at low frequencies and effectively acting to connect said pair of capacitors in parallel with each other to resonate at a low frequency, over a circuit connection provided by said inductance, said inductance loop constituting a short circuit connection between said capacitors of negligible inductance value at said lower frequency.

3. A multi-band vacuum tube circuit adapted to operate in any of a plurality of wavebands, said circuit comprising a vacuum tube having a plurality of electrodes, a connection between one of said electrodes and a point of fixed potential, a first inductor having one end fixedly coupled to another of said electrodes, a first variable tuning capacitor fixedly connected between one end of said inductor and said point of fixed potential, a second variable tuning capacitor fixedly connected between the other end of said inductor and said point of fixed potential, said inductor in combination with said capacitors constituting a tank circuit tunable over a predetermined range of frequencies, a second inductor of much greater inductance than said first inductor, and a waveband-changing means including a switch, said switch being operative in one position of adjustment to connect said second inductor in shunt with one of said variable tuning capacitors to form, in combination with said first inductor and said variable tuning capacitors, a modified tank circuit tunable over a substantially lower range of frequencies than said first-named tank circuit, the inductance of said first inductor being so small compared to the inductance of said second inductor that the reactance of said first inductor is negligibly small throughout said lower frequency range.

4. A multi-band radio frequency circuit for operation over a plurality of Wavebands. comprising a first inductor, a first variable capacitor having one terminal fixedly connected to one terminal of said inductor, a second variable tuning capacitor having one terminal fixedly connected to the other terminal of said inductor, the other terminal of each of said variable capacitors beingconnected to each other, said inductor in combination with said capacitors constituting a tank circuit tunable over a predetermined range of frequencies, a second inductor of considerably reater inductance than said first inductor and a waveband-changing switch for connecting said second inductor in shunt with said variable tuningcapacitors to form therewith a modified tank circuit tunable over a substantially lower-range of frequencies than said first-named tank circuit, said capacitors being connected in shunt with. each other over said first inductor, the inductance of said first inductor being so small compared to the inductance of said second inductor that the reactance of said first inductor acting as a shunt connection for said capacitors is negligibly small throughout said lower frequency range.

JOHN G. CROCKETT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 317,956 Great Britain Aug. 29, 1929 

